a) Field of the Invention
This invention relates to a method for stabilizing treatment of the activity of chromium oxide catalysts used in the production of chlorine by the catalytic oxidation of hydrogen chloride.
b) Description of the Prior Art
The production process of chlorine by the oxidation of hydrogen chloride has been known as Deacon reaction from long ago. The copper catalysts invented by Deacon in 1868 have been regarded to date as exhibiting the most excellent activity, and a number of catalysts have been proposed which comprise copper chloride and potassium chloride with various compounds incorporated as the third component.
However, in order to oxidize hydrogen chloride at an industrially acceptable range of reaction velocity by the use of these catalysts, it is necessary to increase the reaction temperature to 450.degree. C. or higher, which raises such problems as the reduction of catalyst life accompanying the scattering of anyone of catalyst components. Further, the oxidation reaction of hydrogen chloride is an equilibrium reaction and therefore the amount of chlorine formed decreases as the reaction temperature becomes higher. Accordingly, it has become necessary to develop catalysts which are active at a temperature as low as possible.
From these points of view, catalysts other than those of copper have been proposed. However, none of the catalyst has been reported which gives practically acceptable performances.
Chromium oxide catalysts have a higher stability and resistance to a high temperature than that of copper catalysts, so as to be proposed to use it as an oxidizing catalyst of hydrogen chloride. However, none of the report has described that the oxide has sufficient activity. For example, a process is proposed (British Patent No. 584,790), which comprises passing hydrogen chloride at an approximate temperature of 400.degree. C. over a catalyst, prepared by allowing an aqueous chromic anhydride or chromium nitrate solution to impregnate into a suitable carrier followed by calcination, to generate chlorine, in cyclic repetitions of stopping the feed of hydrogen chloride upon deactivation of the catalyst, passing air to regenerate the catalyst, stopping the supply of air and passing again hydrogen chloride over the catalyst.
Further, using a catalyst which is supported dichromate or dark green chromium oxide on its carrier, hydrogen chloride and an oxygen-containing gas are reacted at a temperature of 420.degree.-430.degree. C. and a space velocity of 380 hr.sup.-1 to attain a conversion rate of hydrogen chloride of 67.4% based on the equilibrium value (British Patent No. 676,667). In this case, the conversion is decreased to 63% at a higher space velocity of 680 hr.sup.-1. The reaction may be observed even at a lower temperature of 340.degree. C., in which case a low conversion rate of only 52% is obtained despite a low space velocity as low as 65 hr.sup.-1. Since all of these processes are employing high reaction temperatures and low space velocities, their practical utilization is rather difficult.
On one hand, it has been found that a chromium oxide catalyst obtained by calcining, at a temperature of 800.degree. C. or lower, a compound obtained by reacting an aqueous chromium salt solution with ammonia exhibits a high activity in the oxidation reaction of hydrogen chloride (Japanese Patent Laid-Open No. 275,104/86, U.S. Pat. No. 4,828,815). With this catalyst, it has become possible to produce chlorine at a lower temperature and a higher space time yield than with anyone of the conventional catalyst.
On the other hand, when used in the oxidation reaction of gaseous hydrogen chloride, this catalyst involves such problem that the activity is reduced in a few months after initiation of the reaction. In use of the catalyst, the higher the activity and the longer the life, the greater the industrial advantages.